Nonvolatile semiconductor memory device and method for driving same

ABSTRACT

According to one embodiment, a nonvolatile semiconductor memory device includes a memory unit and a control unit. The memory unit includes a charge storage film and a memory cell transistor. The transistor is provided for each of storage regions configured to store charge in the film. The control unit sets the transistors to an erase threshold by setting erase information in the regions; subsequently sets the transistors to thresholds corresponding to information having n values by programming the information having the n values to at least one of the regions in which the erase information is set; and controls information of at least one storage region before being programmed adjacent to the regions programmed with the information to have a value providing a threshold of the transistor nearer than the erase threshold to the thresholds corresponding to the information having the n values in the state of the transistors provided in the regions being set to the thresholds corresponding to the information having the n values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of application Ser. No.13/017,699 and is based upon and claims the benefit of priority from theprior Japanese Patent Application No. 2010-021670, filed on Feb. 2,2010; the entire contents of each of which are incorporated herein byreference.

FIELD

Embodiments described herein relate generally to a nonvolatilesemiconductor memory device and a method for driving the same.

BACKGROUND

While reducing the dimensions of one element (downscaling) has beennecessary to increase the storage capacity in semiconductor memorydevices (memory), multi-bit storage technology in which two or more bitsof information are stored in one memory cell is also becoming necessaryto store more information.

Improvements to photolithography technology and technology to formmemory cells in three-dimensional structures are being considered todownscale memory cells.

The inventors of the application have proposed collectively patternedthree-dimensionally stacked memory cells that realize athree-dimensional structure using few processes (for example, refer toJP-A 2007-320215 (Kokai)). According to this method, it is possible tosuppress cost increases because it is possible to form a stacked memorycollectively regardless of the number of stacks.

As such downscaling of memory cells progresses, interference betweenadjacent cells greatly affects the reliability of the data. Particularlyin the case where multi-bit information is stored, the reliability isaffected when the thresholds of memory cell transistors corresponding tothe multi-bit information programmed to the memory cells fluctuate dueto the information of adjacent cells; and the data retentioncharacteristics also may be affected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are schematic views illustrating a nonvolatilesemiconductor memory device according to an embodiment;

FIG. 2 is a schematic cross-sectional view illustrating the overallconfiguration of the nonvolatile semiconductor memory device accordingto the embodiment;

FIG. 3 is a schematic perspective view illustrating the configuration ofthe nonvolatile semiconductor memory device according to the embodiment;

FIG. 4 is a schematic cross-sectional view illustrating theconfiguration of a portion of the nonvolatile semiconductor memorydevice according to the embodiment;

FIG. 5 is a schematic plan view illustrating the configuration ofelectrode films of the nonvolatile semiconductor memory device accordingto the embodiment;

FIG. 6 and FIG. 7 are schematic views illustrating the data retentioncharacteristics;

FIGS. 8A and 8B are schematic views illustrating an example of thresholddistributions of a comparative example;

FIGS. 9A and 9B are schematic views illustrating an example of thresholddistributions of a first embodiment;

FIG. 10A to FIG. 11C are schematic views illustrating an example ofthreshold distributions of other examples of the first embodiment;

FIGS. 12A to 12C are schematic views illustrating an example ofthreshold distributions of a second embodiment;

FIGS. 13A to 13D are schematic views illustrating an example of thetransition of the information of memory cells of the second embodiment;

FIG. 14A to FIG. 15D are schematic views illustrating an example of thetransition of the information of the memory cells of a third embodiment;and

FIG. 16 is a circuit diagram illustrating a drive circuit configurationof the nonvolatile semiconductor memory device according to theembodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a nonvolatile semiconductormemory device includes a memory unit and a control unit. The memory unitincludes a charge storage film and a memory cell transistor. The memorycell transistor is provided for each of a plurality of storage regionsconfigured to store charge in the charge storage film. A threshold ofthe memory cell transistor fluctuates due to information set in thestorage region. The control unit is configured to: set the memory celltransistors to an erase threshold by setting erase information in theplurality of storage regions; subsequently set the memory celltransistors provided in the storage regions to thresholds correspondingto information having n (n being an integer not less than 2) values byprogramming the information having the n values to at least one of thestorage regions in which the erase information is set; and controlinformation of at least one storage region before being programmedadjacent to the storage regions programmed with the information to havea value providing a threshold of the memory cell transistor nearer thanthe erase threshold to the thresholds corresponding to the informationhaving the n values in the state of the memory cell transistors providedin the storage regions being set to the thresholds corresponding to theinformation having the n values.

According to another embodiment, a method is disclosed for driving anonvolatile semiconductor memory device. The device includes a chargestorage film and a memory cell transistor. The memory cell transistor isprovided for each of a plurality of storage regions configured to storecharge in the charge storage film. A threshold of the memory celltransistor fluctuates due to information set in the storage regions. Themethod can set the memory cell transistors to an erase threshold bysetting erase information in the plurality of storage regions.Subsequently, the method can set the memory cell transistors provided inthe storage regions to thresholds corresponding to information having n(n being an integer not less than 2) values by programming theinformation having the n values to the storage regions. In addition, themethod can control information of at least one storage region beforebeing programmed adjacent to the storage regions programmed with theinformation to have a value providing a threshold of the memory celltransistor nearer than the erase threshold to the thresholdscorresponding to the information having the n values in the state of thememory cell transistors provided in the storage regions being set to thethresholds corresponding to the information having the n values.

Embodiments of the invention will now be described with reference to thedrawings.

The drawings are schematic or conceptual; and the relationships betweenthe thicknesses and widths of portions, the proportional coefficients ofsizes among portions, etc., are not necessarily the same as the actualvalues thereof. Further, the dimensions and the proportionalcoefficients may be illustrated differently among the drawings, even foridentical portions. In the specification and the drawings of theapplication, components similar to those described in regard to adrawing thereinabove are marked with like reference numerals, and adetailed description is omitted as appropriate.

FIGS. 1A to 1E are schematic views illustrating a nonvolatilesemiconductor memory device according to an embodiment.

Namely, FIG. 1A illustrates an example of threshold distributions ofmemory cell transistors; and FIG. 1B illustrates an example of thetransition of the information of the memory cells.

FIG. 2 is a schematic cross-sectional view illustrating the overallconfiguration of the nonvolatile semiconductor memory device accordingto the embodiment.

FIG. 3 is a schematic perspective view illustrating the configuration ofthe nonvolatile semiconductor memory device according to the embodiment.

For easier viewing of the drawing in FIG. 3, only the conductiveportions are illustrated, and the insulating portions are omitted.

FIG. 4 is a schematic cross-sectional view illustrating theconfiguration of a portion of the nonvolatile semiconductor memorydevice according to the embodiment.

FIG. 5 is a schematic plan view illustrating the configuration ofelectrode films of the nonvolatile semiconductor memory device accordingto the embodiment.

Configuration of the Nonvolatile Semiconductor Memory Device

The nonvolatile semiconductor memory device 110 according to theembodiment of the invention is a three-dimensionally stacked flashmemory.

First, an overview of the configuration of the nonvolatile semiconductormemory device 110 will be described using FIG. 2 to FIG. 5.

As illustrated in FIG. 2, the nonvolatile semiconductor memory device110 includes a memory unit MU.

The nonvolatile semiconductor memory device 110 may further include acontrol unit CTU. The memory unit MU and the control unit CTU areprovided on a major surface 11 a of a semiconductor substrate 11 madeof, for example, monocrystalline silicon. However, the control unit CTUmay be provided on a substrate separate from the substrate on which thememory unit MU is provided. Hereinbelow, the case is described where thememory unit MU and the control unit CTU are provided on the samesubstrate (the semiconductor substrate 11).

In the semiconductor substrate 11, for example, a memory array regionMR, in which memory cells MC are provided, and a peripheral region PRare set. The peripheral region PR is provided in, for example, theperiphery of the memory array region MR. In the peripheral region PR,various peripheral region circuits PR1 are provided on the semiconductorsubstrate 11.

In the memory array region MR, a circuit unit CU, for example, isprovided on the semiconductor substrate 11; and the memory unit MU isprovided on the circuit unit CU. The circuit unit CU is provided asnecessary and may be omitted. An inter-layer insulating film 13 made of,for example, silicon oxide is provided between the circuit unit CU andthe memory unit MU.

At least a portion of the control unit CTU may be provided, for example,in at least one selected from the peripheral region circuit PR1 and thecircuit unit CU recited above.

The memory unit MU includes a matrix memory cell unit MU1 includingmultiple memory cell transistors and an interconnect connection unit MU2that connects the interconnects of the matrix memory cell unit MU1.

FIG. 3 illustrates the configuration of the matrix memory cell unit MU1.

Namely, a portion of the cross section along A-A′ of FIG. 3 and aportion of the cross section along line B-B′ of FIG. 3 are illustratedas the matrix memory cell unit MU1 in FIG. 2.

As illustrated in FIG. 2 and FIG. 3, a stacked structural body ML isprovided on the major surface 11 a of the semiconductor substrate 11 inthe matrix memory cell unit MU1. The stacked structural body ML includesmultiple electrode films WL and multiple inter-electrode insulatingfilms 14 stacked alternately in a direction perpendicular to the majorsurface 11 a.

In specification of the application, an XYZ orthogonal coordinate systemis introduced for convenience of description. In this coordinate system,a direction perpendicular to the major surface 11 a of the semiconductorsubstrate 11 is taken as a Z-axis direction (a first direction). Onedirection in a plane parallel to the major surface 11 a is taken as aY-axis direction (a second direction). A direction perpendicular to theZ axis and the Y axis is taken as an X-axis direction (a thirddirection).

The stacking direction of the electrode film WL and the inter-electrodeinsulating film 14 of the stacked structural body ML is the Z-axisdirection. In other words, the electrode film WL and the inter-electrodeinsulating film 14 are provided parallel to the major surface 11 a. Theelectrode film WL is divided into, for example, erasing block units.

FIG. 4 illustrates the configuration of the matrix memory cell unit MU1and corresponds to, for example, a portion of the cross section alongline B-B′ of FIG. 3.

As illustrated in FIG. 3 and FIG. 4, the memory unit MU of thenonvolatile semiconductor memory device 110 includes the stackedstructural body ML recited above, a semiconductor pillar SP (a firstsemiconductor pillar SP1) which is a semiconductor portion piercing thestacked structural body ML in the Z-axis direction, a charge storagefilm 48, an inner insulating film 42, an outer insulating film 43, andan interconnect WR.

The charge storage film 48 is provided between the semiconductor pillarSP and each of electrode films WL. The inner insulating film 42 isprovided between the charge storage film 48 and the semiconductor pillarSP. The outer insulating film 43 is provided between the charge storagefilm 48 and each of electrode films WL. The interconnect WR iselectrically connected to one end of the semiconductor pillar SP.

In other words, the outer insulating film 43, the charge storage film48, and the inner insulating film 42 are formed in this order on thewall face of the interior of a through-hole TH piercing the stackedstructural body ML in the Z-axis direction; and the semiconductor pillarSP is formed by filling a semiconductor into the remaining space.

The memory cells MC are provided at the intersections between theelectrode films WL and the semiconductor pillars SP of the stackedstructural body ML. In other words, the memory cell transistorsincluding the charge storage film 48 are provided at the portions wherethe electrode films WL and the semiconductor pillars SP intersect in athree-dimensional matrix configuration; and the memory cell transistorsfunction as the memory cells MC that store data by storing charge in thecharge storage film 48. Accordingly, the charge storage film 48 of thememory cells MC at the positions of the electrode films WL function asstorage regions; and multiple storage regions are provided along thecharge storage film 48.

The inner insulating film 42 functions as a tunneling insulating film ofthe memory cell transistor of the memory cell MC. On the other hand, theouter insulating film 43 functions as a blocking insulating film of thememory cell transistor of the memory cell MC. The inter-electrodeinsulating film 14 functions as an inter-layer insulating film thatinsulates the electrode films WL from each other.

The electrode film WL may include any conductive material. For example,amorphous silicon or polysilicon provided with conductivity byintroducing an impurity may be used; and metals, alloys, etc., also maybe used. A prescribed electrical signal is applied to the electrode filmWL; and the electrode film WL functions as a word line of thenonvolatile semiconductor memory device 110.

The inter-electrode insulating film 14, the inner insulating film 42,and the outer insulating film 43 may include, for example, a siliconoxide film. The inter-electrode insulating film 14, the inner insulatingfilm 42, and the outer insulating film 43 may be single-layer films ormay be stacked films.

The charge storage film 48 may include, for example, a silicon nitridefilm and functions as a portion that stores information by storing oremitting charge by an electric field applied between the semiconductorpillar SP and the electrode film WL. The charge storage film 48 may be asingle-layer film or a stacked film.

As described below, the inter-electrode insulating film 14, the innerinsulating film 42, the charge storage film 48, and the outer insulatingfilm 43 are not limited to the materials illustrated above; and anymaterial may be used.

Although the case is illustrated in FIG. 2 and FIG. 3 where the stackedstructural body ML includes four electrode film WL layers, the number ofelectrode films WL provided in the stacked structural body ML isarbitrary. Hereinbelow, the case of four electrode films WL will bedescribed.

In this specific example, two of the semiconductor pillars SP areconnected by a connection portion CP (a connection portion semiconductorlayer).

In other words, the memory unit MU further includes a secondsemiconductor pillar SP2 (the semiconductor pillar SP) and a firstconnection portion CP1 (the connection portion CP).

The second semiconductor pillar SP2 is adjacent to the firstsemiconductor pillar SP1 (the semiconductor pillar SP), for example, inthe Y-axis direction to pierce the stacked structural body ML in theZ-axis direction. The first connection portion CP1 electrically connectsthe first semiconductor pillar SP1 and the second semiconductor pillarSP2 at the same Z-axis direction side (the semiconductor substrate 11side). The first connection portion CP1 is provided to extend in theY-axis direction. The first connection portion CP1 may include the samematerial as the first and second semiconductor pillars SP1 and SP2.

In other words, a back gate BG (a connection portion conductive layer)is provided on the major surface 11 a of the semiconductor substrate 11with the inter-layer insulating film 13 interposed therebetween. Atrench (a trench CTR described below) is provided in the portion of theback gate BG opposing the first and second semiconductor pillars SP1 andSP2; the outer insulating film 43, the charge storage film 48, and theinner insulating film 42 are formed in the interior of the trench; andthe connection portion CP made of a semiconductor is filled into theremaining space. The formation of the outer insulating film 43, thecharge storage film 48, the inner insulating film 42, and the connectionportion CP in the trench recited above may be performed collectively andsimultaneously with the formation of the outer insulating film 43, thecharge storage film 48, the inner insulating film 42, and thesemiconductor pillar SP in the through-hole TH. Thus, the back gate BGis provided opposing the connection portion CP.

Thereby, a semiconductor pillar having a U-shaped configuration isformed of the first and second semiconductor pillars SP1 and SP2 and theconnection portion CP to form a NAND string having a U-shapedconfiguration.

Although the connection portion CP includes the function of electricallyconnecting the first and second semiconductor pillars SP1 and SP2, theconnection portion CP also may be utilized as one memory cell. Thereby,more storage bits can be provided. Hereinbelow, the case is describedwhere the connection portion CP electrically connects the first andsecond semiconductor pillars SP1 and SP2 and is not used as a storageunit. In such a case, although the charge storage film 48 opposing theconnection portion CP does not function as a storage unit, the portionof the charge storage film 48 opposing the connection portion CP also isreferred to as a “storage layer” to simplify the description.

As illustrated in FIG. 2 and FIG. 3, the end of the first semiconductorpillar SP1 opposite to the first connection portion CP1 is connected toa bit line BL (a second interconnect W2); and the end of the secondsemiconductor pillar SP2 opposite to the first connection portion CP1 isconnected to a source line SL (a first interconnect W1). Thesemiconductor pillar SP and the bit line BL are connected by a via V1and a via V2. The interconnect WR includes the first interconnect W1 andthe second interconnect W2.

In this specific example, the bit line BL extends in the Y-axisdirection; and the source line SL extends in the X-axis direction.

Between the stacked structural body ML and the bit line BL, a drain-sideselection gate electrode SGD (a first selection gate electrode SG1,i.e., the selection gate electrode SG) is provided opposing the firstsemiconductor pillar SP1; and a source-side selection gate electrode SGS(a second selection gate electrode SG2, i.e., the selection gateelectrode SG) is provided opposing the second semiconductor pillar SP2.Thereby, the desired data can be the programmed to or read from anymemory cell MC of any semiconductor pillar SP.

The selection gate electrode SG may include any conductive material. Forexample, polysilicon or amorphous silicon may be used. In this specificexample, the selection gate electrode SG is divided in the Y-axisdirection and has a band configuration extending along the X-axisdirection.

As illustrated in FIG. 2, an inter-layer insulating film 15 is providedin the uppermost portion (the side most distal to the semiconductorsubstrate 11) of the stacked structural body ML. An inter-layerinsulating film 16 is provided on the stacked structural body ML; theselection gate electrode SG is provided thereon; and an inter-layerinsulating film 17 is provided between the selection gate electrodes SG.A through-hole is provided in the selection gate electrode SG; aselection gate insulating film SGI of the selection gate transistor isprovided on an inner side face thereof; and a semiconductor is filledtherein. This semiconductor continues from the semiconductor pillar SP.In other words, the memory unit MU further includes the selection gateelectrode SG stacked on the stacked structural body ML in the Z-axisdirection and pierced by the semiconductor pillar SP at the interconnectWR (at least one selected from the source line SL and the bit line BL)side.

An inter-layer insulating film 18 is provided on the inter-layerinsulating film 17; the source line SL and a via 22 (the vias V1 and V2)are provided thereon; and an inter-layer insulating film 19 is providedaround the source line SL. An inter-layer insulating film 23 is providedon the source line SL; and the bit line BL is provided thereon. The bitline BL has a band configuration along the Y-axis direction.

The inter-layer insulating films 15, 16, 17, 18, 19, and 23 and theselection gate insulating film SGI may include, for example, siliconoxide.

Herein, the semiconductor pillar is multiply provided in the nonvolatilesemiconductor memory device 110. “Semiconductor pillar SP” is used torefer to all of the semiconductor pillars or any semiconductor pillar;and “nth semiconductor pillar SPn” (n being any integer not less than 1)is used to refer to a designated semiconductor pillar when describing arelationship between semiconductor pillars, etc.

For the electrode films WL as illustrated in FIG. 5, the electrode filmscorresponding to the semiconductor pillars SP(4 m+1) and SP(4 m+4) havea common connection to form an electrode film WLA, where m is an integernot less than 0 and n is (4 m+1) and (4 m+4); and the electrode filmscorresponding to the semiconductor pillars SP(4 m+2) and SP(4 m+3) havea common connection to form an electrode film WLB, where n is (4 m+2)and (4 m+3). In other words, the electrode films WL have a configurationin which the electrode film WLA and the electrode film WLB are combinedwith each other in a comb teeth configuration opposing in the X-axisdirection.

As illustrated in FIG. 4 and FIG. 5, the electrode film WL is divided byan insulating layer IL; and the electrode film WL is divided into afirst region (the electrode film WLA) and a second region (the electrodefilm WLB).

As in the interconnect connection unit MU2 illustrated in FIG. 2, theelectrode film WLB is connected at one X-axis direction end to a wordinterconnect 32 by a via plug 31 and is electrically connected to, forexample, a drive circuit provided in the semiconductor substrate 11.Similarly, the electrode film WLA is connected at the other X-axisdirection end to the word interconnect by the via plug and iselectrically connected to the drive circuit. In other words, the lengthin the X-axis direction of each of the electrode films WL (the electrodefilm WLA and the electrode film WLB) stacked in the Z-axis directionchanges in a stairstep configuration; and the electrode films WL areelectrically connected to the drive circuit by the electrode film WLA atthe one X-axis direction end and by the electrode film WLB at the otherX-axis direction end.

As illustrated in FIG. 3, the memory unit MU may further include a thirdsemiconductor pillar SP3 (the semiconductor pillar SP), a fourthsemiconductor pillar SP4 (the semiconductor pillar SP), and a secondconnection portion CP2 (the connection portion CP).

The third semiconductor pillar SP3 is adjacent to the secondsemiconductor pillar SP2 in the Y-axis direction on the side of thesecond semiconductor pillar SP2 opposite to the first semiconductorpillar SP1 and pierces the stacked structural body ML in the Z-axisdirection. The fourth semiconductor pillar SP4 is adjacent to the thirdsemiconductor pillar SP3 in the Y-axis direction on the side of thethird semiconductor pillar SP3 opposite to the second semiconductorpillar SP2 and pierces the stacked structural body ML in the Z-axisdirection.

The second connection portion CP2 electrically connects the thirdsemiconductor pillar SP3 and the fourth semiconductor pillar SP4 on thesame side (the same side as the first connection portion CP1) in theZ-axis direction. The second connection portion CP2 is provided toextend in the Y-axis direction to oppose the back gate BG.

The charge storage film 48 is provided between the third semiconductorpillar SP3 and each of the electrode films WL, between the fourthsemiconductor pillar SP4 and each of the electrode films WL, and betweenthe back gate BG and the second connection portion CP2. The innerinsulating film 42 is provided between the third semiconductor pillarSP3 and the charge storage film 48, between the fourth semiconductorpillar SP4 and the charge storage film 48, and between the chargestorage film 48 and the second connection portion CP2. The outerinsulating film 43 is provided between the charge storage film 48 andeach of the electrode films WL and between the charge storage film 48and the back gate BG.

The source line SL is connected to a third end portion of the thirdsemiconductor pillar SP3 on the side opposite to the second connectionportion CP2. The bit line BL is connected to a fourth end portion of thefourth semiconductor pillar SP4 on the side opposite to the secondconnection portion CP2.

The source-side selection gate electrode SGS (the third selection gateelectrode SG3, i.e., the selection gate electrode SG) is provided tooppose the third semiconductor pillar SP3. The drain-side selection gateelectrode SGD (the fourth selection gate electrode SG4, i.e., theselection gate electrode SG) is provided to oppose the fourthsemiconductor pillar SP4.

The embodiments will now be described. One feature of the nonvolatilesemiconductor memory device 110 according to the embodiments regards thethreshold of the memory cell transistor being set in the erasingoperation and the programming operation of the memory cells by thecontrol unit CTU. Hereinbelow, the embodiments are described with afocus on the operations of the threshold setting by the control unitCTU. Although the invention may be applied to technology that recordsinformation having n (n being an integer not less than 2) values in onememory cell, an example is used hereinbelow in which information havingn=4, i.e., four-valued information, is recorded to simplify thedescription. The four-valued information is the two-bit data “11,” “10,”“01,” and “00.”

The recording of the four-valued information in the memory cells isperformed by injecting charge having amounts according to thefour-valued information into the storage region of each of the memorycells. Because the threshold of the memory cell transistor fluctuatesaccording to the amount of the charge of the storage region, it is readwhether one of the four values is stored by applying to the memory celltransistor a prescribed voltage that can identify the four values andfrom the operating state at that time.

In the case where the four-valued information is recorded in the memorycells, the thresholds of the memory cell transistors can have fourdistributions according to the four-valued information. In the casewhere the information of the memory cells is erased, the memory celltransistors are set to the erase threshold. The erase threshold also canhave a distribution in a certain range. In the description hereinbelow,these thresholds are illustrated using symbols A, B, C, D, and E. In thedescription hereinbelow, the threshold of the memory cell transistoralso may be referred to as simply “the threshold of the memory cell.”

First Embodiment

The configuration of a nonvolatile semiconductor memory device accordingto a first embodiment may be applied not only to the configuration ofthe nonvolatile semiconductor memory device according to the embodimentdescribed using FIG. 2 to FIG. 5 but also to the configuration of anonvolatile semiconductor memory device which has no connection portionand no back gate BG and includes NAND strings having I-shapedconfigurations in which each of the semiconductor pillars is independentand the configuration of a planar nonvolatile semiconductor memorydevice in which a MONOS structure is provided in a planar configuration.

As illustrated in FIG. 1A, the control unit CTU (referring to FIG. 2) isconfigured to set the thresholds of memory cell transistors Tr accordingto the operations of the erase threshold E and the thresholds A to Dcorresponding to the four-valued information. In other words, thecontrol unit CTU uses n+1 thresholds when performing the recording ofthe information having the n values.

Of the distributions of these thresholds as illustrated in FIG. 1A, allof the values of the distribution of the erase threshold E from theupper limit to the lower limit are on the negative polarity side. On theother hand, all of the values of the distributions of the thresholds Ato D corresponding to each of the four-valued information are on thepositive polarity side.

It is favorable for the widths of the distributions of the thresholds Ato D to be set to be narrower than the width of the distribution of thethreshold E. In other words, it is necessary for the distributions ofthe thresholds A to D for representing the four-valued information to bereliably contained in one of the distributions of the thresholds A to Dto suppress erroneous detection during the read out. To this end, theprogramming of the information to the memory cells includes performing averify operation of the programming that repeatedly programs until thememory cell transistors reach a threshold voltage not less than aprogram verify voltage. As long as it is possible to determine thefour-valued information, the verify operation of the programming may notbe performed.

On the other hand, the erasing operation of the memory cells isperformed collectively to erase the information of multiple memorycells. At this time, while the thresholds of the memory cell transistorsare set to be negative, a wide distribution can be used because it isunnecessary to discriminate within the erase information. Therefore, inthe erasing operation which takes more time compared to the programmingof the information, the verify operations of the erasing in which theerasing is performed repeatedly until the memory cell transistors reacha threshold voltage not more than an erase verification voltage can befewer than the verify operations of the programming. As long as it ispossible to determine the erase information, the verify operation of theerasing may not be performed.

FIG. 1B illustrates the transition of the information according to theoperations of the control unit CTU. This drawing illustrates a string inwhich multiple memory cells are connected in series. This drawingillustrates an equivalent circuit to describe this embodiment; and thestring is simplified for description by being illustrated as a circuitextending in one direction. This does not represent the actualconfiguration. Restated, all three-dimensional memory, so-called planarNAND in which a MONOS structure is provided on a plane, and anyconfiguration which can be illustrated similarly by an equivalentcircuit can be applied to this embodiment.

A storage region ME and the memory cell transistor Tr are provided ineach of the memory cells. A selection transistor controlled by thesource-side selection gate electrode SGS is connected to one end of thesuccessive memory cell transistors Tr; and a selection transistorcontrolled by the drain-side selection gate electrode SGD is connectedto the other end.

The control unit CTU is configured to perform “collective erasing,”“programming,” “A-programming,” and then “additional programming” inthis order from above as illustrated in FIG. 1B.

First, the control unit CTU performs the collective erasing of themultiple memory cells of the string. By the erasing operation, thethresholds of the multiple memory cell transistors Tr are set to thethreshold E. At this time, this is a state in which the width of thedistribution of the threshold E is relatively wide.

Then, the control unit CTU performs the programming of the informationto the cells to be programmed of the string. The programming isperformed, for example, in order from the memory cell on the sourceside. In the memory cells to be programmed, a high voltage is applied tothe tunneling oxide film; and an amount of electrons corresponding tothe four-valued information is injected into the storage region. On theother hand, in the memory cells not to be programmed, a high voltage isnot applied to the tunneling oxide film; and electrons are not injectedinto the storage region. The programming is not always limited to thecase of being performed from the memory cell on the source side and maybe performed from the memory cell on the drain side or torandomly-accessed memory cells.

By the programming of the information, the thresholds of the memory celltransistors Tr of the cells to be programmed are set to one selectedfrom the thresholds A to D corresponding to the four-valued information.In other words, in the memory cells in which the information of “11”from the four values is programmed, the thresholds of the memory celltransistors Tr are set to the threshold A. In the memory cells in whichthe information of “01” from the four values is programmed, thethresholds of the memory cell transistors Tr are set to the threshold Bwhich is higher than the distribution of the threshold A. In the memorycells in which the information of “10” from the four values isprogrammed, the thresholds of the memory cell transistors Tr are set tothe threshold C which is higher than the distribution of the thresholdB. In the memory cells in which the information of “00” from the fourvalues is programmed, the thresholds of the memory cell transistors Trare set to the threshold D which is higher than the distribution of thethreshold C.

As described above, because all of the values of the distributions ofthe thresholds A to D corresponding to the four-valued information areon the positive polarity side, all of the values of the thresholds ofthe memory cell transistors Tr of the cells to be programmed are set tobe on the positive polarity side.

In the programming, it is favorable for the verify operation of theprogramming to be performed such that the thresholds of the memory celltransistors Tr of the cells to be programmed are contained in thedistributions of the thresholds A to D corresponding to the four-valuedinformation. As long as it is possible to determine the four-valuedinformation, the verify operation of the programming may not beperformed. Thereby, the widths of the distributions of the thresholds Ato D are narrower than the distribution of the erase threshold E; andthe four-valued information can be discriminated reliably. Then, thecontrol unit CTU performs the A-programming.

The A-programming is processing to program information to at least onecell before being programmed adjacent to the programmed cells such thatthe threshold of the memory cell transistor becomes the threshold A.Here, the at least one cell before being programmed adjacent to theprogrammed cells is referred to as the “adjacent cell.”

Although the case of one adjacent cell is illustrated in FIG. 1B, theadjacent cell may be two or more or may be all of the cells before beingprogrammed. The A-programming may be executed as a programming separatefrom the programming performed previously or may continue from theprogramming to the cells to be programmed as an extension of theprogramming performed previously to program information to the adjacentcell.

The A-programming is an operation that programs the information to theadjacent cell to provide a value nearer than the distribution of theerase threshold E to the thresholds A to D corresponding to thefour-valued information. Of the thresholds A to D corresponding to thefour-valued information in the example illustrated in FIG. 1B, thethreshold A having the lowest voltage is utilized as this value.

In the case where the programming is performed to randomly-accessedmemory cells as illustrated in FIG. 1C, the A-programming may beperformed to the adjacent cells on either side of the programmed cells(the cells to be programmed).

By the A-programming, the difference of the set thresholds, i.e., thedifference of the charge, between the memory cell at the end of thecells to be programmed and the cell before being programmed adjacent tothe memory cell is less than in the state directly after theprogramming.

Directly after the programming in the example illustrated in FIG. 1B,the set threshold level of the memory cell at the end of the cells to beprogrammed is C; and the threshold of the cell before being programmedadjacent thereto is E, i.e., as-is in the erased state. Thereby, thedifference of the thresholds between the memory cell at the end of thecells to be programmed and the adjacent cell is C-E.

By performing the A-programming from this state, the thresholddistribution of the adjacent cell becomes from the distribution of thethreshold E to the distribution of the threshold A. Thereby, thedifference of the thresholds between the memory cell at the end of thecells to be programmed and the adjacent cell becomes C-A. Accordingly,after the A-programming, the difference of the thresholds between thememory cell at the end of the cells to be programmed and the adjacentcell is reduced from that prior to the A-programming by an amount ofA-E. Thus, by the difference of the thresholds being reduced, the memorycell at the end of the cells to be programmed is less susceptible toeffects from the adjacent cell, which leads to better data retentioncharacteristics.

Then, the control unit CTU performs the additional programming. Theadditional programming is processing to program the desired four-valuedinformation to cells to be programmed in a latter stage and includes theadjacent cell to which the A-programming was performed. The additionalprogramming can be executed by a method similar to the programming.Here, because the threshold of the memory cell transistor Tr of theadjacent cell to which the A-programming was performed is thedistribution of the threshold A, this cell is reprogrammed to oneselected from the thresholds A to D of the four values.

Because the threshold A which has the lowest voltage of the four-valuedinformation is utilized in the A-programming, it is sufficient for theprocessing to increase the charge injected into the storage region whenreprogramming to the other information (B to D). That is, the additionalprogramming of the desired information can be performed as-is withoutperforming the erasing operation. In the case where additionalprogramming of the information corresponding to the threshold A isperformed to the adjacent cell, it is unnecessary to program anything.

In the case where the thresholds B to D are programmed as substitute forthe A-programming, it is necessary to perform an operation toselectively change the cells for which the additional programming is tobe performed to the threshold E.

FIG. 1D illustrates the transition of the information in the case wherethe threshold D is used as substitute for the A-programming.

FIG. 1E illustrates the operation of changing only the cell to beprogrammed to the threshold E.

In the example illustrated in FIG. 1D, the threshold D, for example, isprogrammed to the adjacent cell of the cells to be programmed assubstitute for the A-programming. In such a case, the adjacent cell ischanged to the threshold E prior to the additional programming.Subsequently, the additional programming to the adjacent cell isperformed.

Specifically, as illustrated in FIG. 1E, a reference potential (e.g., agrounding potential GND) is applied to only the adjacent cell to bechanged to the threshold E; and the other cells are floating or anintermediate potential VeraNS is applied. The intermediate potentialVeraNS is a potential between the reference potential (e.g., thegrounding potential GND) and a potential Vera for erasing. Thereby, onlythe memory cell transistor Tr of the cell to be changed (the adjacentcell) fixed at the reference potential is changed to the threshold E.

Data Retention Characteristics

Data retention characteristics will now be described. FIG. 6 and FIG. 7are schematic views illustrating the data retention characteristics.

These drawings are enlarged views of a memory cell portion, andillustrate only the portion of memory cells MC(1) to (3) correspondingto three electrode films WL on one side of one semiconductor pillar SP.The movement of electrons and electron holes are illustratedschematically.

In one memory cell MC, the charge storage film 48 at the position of theelectrode film WL is the storage region ME. The programming is performedby at least one selected from injecting electrons into the storageregion ME and removing electron holes from the storage region ME. In theexample illustrated in FIG. 6 and FIG. 7, electrons are injected intothe storage region ME during the programming. The erasing is performedby at least one selected from removing electrons from the storage regionME and injecting electron holes into the storage region ME. In theexample illustrated in FIG. 6 and FIG. 7, electron holes are injectedinto the storage region ME during the erasing.

In the example illustrated in FIG. 6, the memory cell MC(2) of thecentral portion of the drawing is in a programmed state in whichelectrons are injected into the storage region ME(2). On the other hand,the memory cell MC(1) and the memory cell MC(3) adjacent thereto are inan erased state in which electron holes are injected into the storageregion ME(1) and the storage region ME(3).

In the memory cell MC(2) which is in the programmed state, the thresholdof the memory cell transistor is set according to the amount of theelectrons injected into the storage region ME(2). In such a state, inthe case where electron holes are stored in the storage regions ME(1)and (3) of the memory cells MC(1) and (3) adjacent to the memory cellMC(2), the electrons stored in the storage region ME(2) of the memorycell MC(2) are easily attracted toward the adjacent memory cells MC(1)and (3).

In other words, the state in which the polarity of the charge stored inthe adjacent memory cells MC(1) and (3) is opposite to that of thecharge stored in the memory cell MC(2), which is in the programmedstate, can be said to be a state in which electrons are easily removedfrom the storage region ME(2) of the memory cell MC(2).

In the case where the electrons stored in the storage region ME(2) areundesirably removed as in the example illustrated in FIG. 6, thethreshold of the memory cell transistor of the memory cell MC(2)undesirably fluctuates from the initial value, which leads to theerroneous detection of the information during the read out. It isconceivable that this may harm the data retention characteristics.

Although the memory cell MC(2) of the central portion of the drawing inthe example illustrated in FIG. 7 is in the programmed state in whichelectrons are injected into the storage region ME(2) similarly to theexample illustrated in FIG. 6, the memory cell MC(1) and the memory cellMC(3) adjacent thereto are in states in which a slight amount ofelectrons are injected into the storage region ME(1) and the storageregion ME(3).

That is, in the case where the polarity of the charge stored in theadjacent memory cells MC(1) and (3) is the same as that of the chargestored in the memory cell MC(2), which is in the programmed state,electrons are not easily removed from the storage region ME(2) of thememory cell MC(2).

The ease or difficulty of removing electrons from the memory cell MC(2),which is in the programmed state, toward the adjacent memory cells MC(1)and (3) may be considered to be relative. Accordingly, electrons areeasier to remove as the potential difference due to the amount of chargebetween the storage region ME(2) of the memory cell MC(2) and thestorage regions ME(1) and (3) of the adjacent memory cells MC(1) and (3)increases and are more difficult to remove as the potential differencedecreases.

In the nonvolatile semiconductor memory device 110 according to thisembodiment, the state illustrated in FIG. 7 is made by performing theA-programming to the adjacent cell by applying a principle such as thatrecited above regarding the relationship between the programmed cellsand the at least one cell before being programmed adjacent thereto (theadjacent cell). Thereby, the potential difference with the programmedcells can be lower than that of the state prior to the programming; anda state can be obtained in which the electrons are not easily removedfrom the programmed cells.

COMPARATIVE EXAMPLE

FIGS. 8A and 8B are schematic views illustrating an example of thethreshold distributions of a comparative example.

In the comparative example, the four-valued information is programmed tothe memory cells; and the erase threshold E and the thresholds A to C ofthe information to be programmed are utilized. In other words, a totalof four thresholds are utilized; and the thresholds representing thefour-valued information include the erase threshold E and the thresholdsA to C.

In the comparative example, “11” from the four-valued information isrepresented by the erase threshold E, “01” is represented by thethreshold A, “10” is represented by the threshold B, and “00” isrepresented by the threshold C.

First, as illustrated in FIG. 8A, the collective erasing of theinformation of the multiple memory cells is performed. By the erasingoperation, the thresholds of the memory cell transistors are set to thethreshold E. At this time, this is a state in which the distribution ofthe threshold E is on the negative polarity side and the distributionwidth thereof is relatively wide.

Then, as illustrated in FIG. 8B, the programming of the information isperformed to the cells to be programmed. By the programming of theinformation, the thresholds of the memory cell transistors of the cellsto be programmed become one selected from the thresholds correspondingto the four-valued information.

Here, because “11” from the four values is represented by the erasethreshold E, the programming is substantially not performed for thecells to be programmed when programming “11.” In the memory cells towhich the information of “01” from the four values is programmed, thethresholds of the memory cell transistors are set to the threshold A ofthe positive polarity side.

In the memory cells to which the information of “10” from the fourvalues is programmed, the thresholds of the memory cell transistors areset to the threshold B which is higher than the distribution of thethreshold A. In the memory cells to which the information of “00” fromthe four values is programmed, the thresholds of the memory celltransistors are set to the threshold C which is higher than thedistribution of the threshold B.

In such a comparative example, because the thresholds of the memory celltransistors of the cells before being programmed are set to the erasethreshold E (the negative pole), the polarity of the stored chargebetween the adjacent cells of the programmed cells and the cells beforebeing programmed are opposite in the case where the memory celltransistors of the programmed cells are set to the thresholds A to C(the positive pole); and this is a state in which electrons are easilyremoved from the storage regions of the programmed cells (referring toFIG. 6).

Further, in the programmed cells of the comparative example, in the casewhere, for example, the distribution of the erase threshold Erepresenting “11” is adjacent to the distribution of the thresholds A toC representing the others, the polarity of the stored charge betweenadjacent cells is opposite similarly to those recited above; and this isa state in which the removal of the electrons may occur.

Threshold Distribution of the First Embodiment

FIGS. 9A and 9B are schematic views illustrating an example of thresholddistributions of the first embodiment.

Compared to the comparative example described above, the thresholddistributions illustrated in FIGS. 9A and 9B utilize the thresholds A toD of the information to be programmed in addition to the erase thresholdE. In other words, a total of five thresholds are utilized; and thethresholds representing the four-valued information include thethresholds A to D and do not include the erase threshold E.

For the four-valued information, “11” is represented by the threshold A,“01” is represented by the threshold B, “10” is represented by thethreshold C, and “00” is represented by the threshold D.

First, as illustrated in FIG. 9A, the collective erasing of the multiplememory cells is performed. By the erasing operation, the thresholds ofthe memory cell transistors are set to the threshold E. At this time,this is a state in which the distribution of the threshold E is on thenegative polarity side and the distribution width thereof is relativelywide.

Continuing as illustrated in FIG. 9B, the programming of the informationis performed to the cells to be programmed. By the programming of theinformation, the thresholds of the memory cell transistors of the cellsto be programmed become one selected from the thresholds correspondingto the four-valued information. In the example illustrated in FIG. 9B,“11” from the four values is set in the distribution of the threshold A,“01” is set in the distribution of the threshold B, “10” is set in thedistribution of the threshold C, and “00” is set to the threshold D.

Then, after the desired information is programmed to the cells to beprogrammed, the information providing the threshold A is programmed toat least one cell before being programmed (the adjacent cell) adjacentto the cells to be programmed (the A-programming operation). Thereby,the thresholds in the cells to be programmed and the adjacent cell arecontained in one selected from A to D; and all of these are thresholddistributions of the positive polarity side. Accordingly, the storedcharge of the programmed cells has the same polarity as that of all ofthe cells adjacent thereto; and this is a state in which the electronsare not easily removed from the storage regions of the programmed cells(referring to FIG. 7).

Threshold Distributions of Another Example (Part 1) of the FirstEmbodiment

FIGS. 10A and 10B are schematic views illustrating an example ofthreshold distributions of another example (part 1) of the firstembodiment.

Although the thresholds A to D of the information to be programmed areutilized in addition to the erase threshold E in this example similarlyto the threshold distributions illustrated in FIGS. 9A and 9B, thisexample differs in that the width of the distribution of the threshold Ais wide.

First, as illustrated in FIG. 10A, the collective erasing of theinformation of the multiple memory cells is performed. By the erasingoperation, the thresholds of the memory cell transistors are set to thethreshold E. At this time, this is a state in which the distribution ofthe threshold E is on the negative polarity side and a width We of thedistribution is relatively wide.

Then, as illustrated in FIG. 10B, the programming of the information isperformed to the cells to be programmed. By the programming of theinformation, the thresholds of the memory cell transistors of the cellsto be programmed become one selected from the thresholds correspondingto the four-valued information. In the example illustrated in FIG. 10B,“11” from the four values is set in the distribution of the threshold A,“01” is set in the distribution of the threshold B, “10” is set in thedistribution of the threshold C, and “00” is set in the distribution ofthe threshold D.

Here, when setting to the threshold A representing “11,” a width Wa ofthe distribution of the threshold A is set to be narrower than the widthWe of the distribution of the erase threshold E and wider than widthsWb, Wc, and Wd of the distributions of the thresholds B, C, and D.Specifically, when setting to the threshold A, the verify operations ofthe programming are more than the verify operations of the erasing andfewer than the verify operations of the programming of the thresholds Bto D.

Then, after programming the desired information to the cells to beprogrammed, the information providing the threshold A is programmed tothe at least one cell before being programmed (the adjacent cell)adjacent to the cells to be programmed (the A-programming operation).Thereby, this state is a state in which all of the distributions of thethresholds in the cells to be programmed and the adjacent cell are onthe positive polarity side and the electrons are not easily removed fromthe storage regions of the programmed cells (referring to FIG. 7). Also,in the example illustrated in FIGS. 10A and 10B, because the width Wa ofthe distribution of the threshold A is wider than the widths Wb to Wd ofthe distributions of the thresholds B to D in the programming operationand the A-programming operation, the processing time of each of theoperations is shorter than that of the example illustrated in FIGS. 9Aand 9B.

Threshold Distributions of Another Example (Part 2) of the FirstEmbodiment

FIGS. 11A to 11C are schematic views illustrating an example ofthreshold distributions of another example (part 2) of the firstembodiment.

In this example, although the thresholds A to D of the information to beprogrammed are utilized in addition to the erase threshold E similarlyto the threshold distributions illustrated in FIGS. 10A and 10B, thisexample differs in that the distribution of the threshold A is adistribution straddling 0 V.

First, as illustrated in FIG. 11A, the collective erasing of theinformation of the multiple memory cells is performed. By the erasingoperation, the thresholds of the memory cell transistors are set to thethreshold E. At this time, this is a state in which the distribution ofthe threshold E is on the negative polarity side and the distributionwidth is relatively wide.

Then, as illustrated in FIG. 11B, the programming of the information isperformed to the cells to be programmed. By the programming of theinformation, the thresholds of the memory cell transistors of the cellsto be programmed become one selected from the thresholds correspondingto the four-valued information. In the example illustrated in FIG. 11B,“11” from the four values is set in the distribution of the threshold A,“01” is set in the distribution of the threshold B, “10” is set in thedistribution of the threshold C, and “00” is set in the distribution ofthe threshold D.

Here, when setting the threshold A representing “11,” the distributionof the threshold A is a distribution straddling 0 V. Similarly to theexample illustrated in FIGS. 10A and 10B, the distribution of thethreshold A may be narrower than the width of the distribution of theerase threshold E and wider than the widths of the distributions of thethresholds B to D. Also, the widths of the distributions of thethresholds B to D may be the same. In either case, the distribution ofthe threshold A straddles 0 V.

Then, after programming the desired information to the cells to beprogrammed, the information providing the threshold A is programmed tothe at least one cell before being programmed (the adjacent cell)adjacent to the cells to be programmed (the A-programming operation). Inthis example, the threshold distributions of the cells to be programmedand the adjacent cell are almost entirely on the positive polarity side.However, only a portion of the distribution of the threshold A is athreshold of the negative polarity side. Even in the case where aportion of the distribution of the threshold A is a threshold of thenegative polarity side, a state can be obtained in which the electronsare not easily removed from the storage regions of the programmed cellscompared to the case where the adjacent cell has the distribution of thethreshold E because the threshold is more to the positive polarity sidethan is the distribution of the erase threshold E.

It is not always necessary for the entire distribution of the erasethreshold E to be on the negative polarity side. For example, asillustrated in FIG. 11C, a portion of the distribution of the erasethreshold E may be on the negative polarity side and the other portionmay be on the positive polarity side. Also, as illustrated in FIG. 11B,a portion of the distribution of the erase threshold E on the positivepolarity side may overlap a portion of the distribution of the thresholdA on the threshold E side.

Second Embodiment

An example of a nonvolatile semiconductor memory device according to asecond embodiment will now be described. The configuration of thenonvolatile semiconductor memory device according to the secondembodiment may be applied not only to the configuration of thenonvolatile semiconductor memory device according to the embodimentdescribed using FIG. 2 to FIG. 5 but also to the configuration of anonvolatile semiconductor memory device which has no connection portionand no back gate BG and includes NAND strings having I-shapedconfigurations in which each of the semiconductor pillars is independentand the configuration of a planar nonvolatile semiconductor memorydevice in which a MONOS structure is provided in a planar configuration.The descriptions hereinbelow focus on points different from theoperations of the nonvolatile semiconductor memory device according tothe first embodiment.

FIGS. 12A to 12C are schematic views illustrating an example ofthreshold distributions of the second embodiment. FIGS. 13A to 13D areschematic views illustrating an example of the transition of theinformation of the memory cells. These drawings illustrate an equivalentcircuit to describe this embodiment; and the string is simplified fordescription by being illustrated as a circuit extending in onedirection. This does not represent the actual configuration.

The threshold distributions of the second embodiment correspond toprogramming four-valued information to the memory cells, where athreshold E′ shifted from the erase threshold E toward the positivepolarity side and the thresholds A to C of the information to beprogrammed are utilized. In other words, a total of four thresholds areutilized; and the thresholds representing the four-valued informationinclude the threshold E′ and the thresholds A to C.

For the four-valued information, “11” is represented by the thresholdE′, “01” is represented by the threshold A, “10” is represented by thethreshold B, and “00” is represented by the threshold C.

First, as illustrated in FIG. 12A, the collective erasing of theinformation of the multiple memory cells is performed. By the erasingoperation, the thresholds of the memory cell transistors are set to thethreshold E. At this time, this is a state in which the distribution ofthe threshold E is on the negative polarity side and the width We of thedistribution is relatively wide.

FIG. 13A illustrates the state of the information of the memory cellswhen the collective erasing is performed.

As illustrated in this drawing, in the case where the collective erasingof the multiple memory cells is performed in a string in which themultiple memory cells are connected in series, all of the thresholds ofthe multiple memory cell transistors Tr are set in the distribution ofthe threshold E.

Then, as illustrated in FIG. 12B, the information of the multiple memorycells is set to the threshold E′. The threshold E′ is more toward thepositive polarity side than is the distribution of the erase threshold Eand represents the information from the four-valued information thatprovides a threshold nearest to the distribution of the erase thresholdE, i.e., in this case, “11.” A width We′ of the distribution of thethreshold E′ may be narrower than the width We of the distribution ofthe erase threshold E, may be wider than the widths of the distributionsof the thresholds A to C, or may have the same width as thedistributions of the thresholds A to C.

Although the distribution of the threshold E′ is set to straddle 0 V inthe example illustrated in FIG. 12B, the entire distribution may be setto be on the positive polarity side. By such an operation, thethresholds of the multiple memory cells are set to E′.

FIG. 13B illustrates the state of the information of the memory cellswhen the programming (the E′-programming) of the information providingthe threshold E′ is performed.

As illustrated in this drawing, by performing the E′-programming, all ofthe thresholds of the multiple memory cell transistors Tr are set in thedistribution of the threshold E′. In such a case, the increase of thenumber of the verify operations is suppressed and the E′-programming canbe completed in a relatively short period of time because the width We′of the distribution of the threshold E′ is narrower than the width We ofthe distribution of the erase threshold E and wider than the widths ofthe distributions of the thresholds A to C.

Then, as illustrated in FIG. 12C, the programming of the information isperformed to the cells to be programmed. By the programming of theinformation, the thresholds of the memory cell transistors of the cellsto be programmed become one selected from the distribution of thethresholds corresponding to the four-valued information. Here, because“11” from the four values is represented by the threshold E′, theprogramming is substantially not performed for the cells to beprogrammed when programming the “11.” In the memory cells in which theinformation of “01” from the four values is programmed, the thresholdsof the memory cell transistors are set in the distribution of thethreshold A of the positive polarity side. Thereby, the thresholds ofthe programmed cells are set to E′ or one selected from A to C.

The state of the information of the memory cells when the programming isperformed is illustrated in FIG. 13C.

By performing the programming as illustrated in this drawing, thethresholds of the memory cell transistors Tr of the programmed cells areset to the distribution of the threshold E′ or one selected from thethresholds A to C. Because “11” from the four values is represented bythe threshold E′, the programming is substantially not performed for thecells to be programmed when programming the “11.”

In the state in which the information is programmed, all of thethresholds of the memory cell transistors Tr of the cells before beingprogrammed including the adjacent cell adjacent to the programmed cellsare the threshold E′ as-is in the state of the previous E′-programming.

Here, focusing on the memory cell at the end of the cells to beprogrammed and the cell before being programmed adjacent to the memorycell at the end, the difference of the set thresholds is a differencehaving the threshold E′ of the adjacent cell as a reference. In otherwords, the difference is smaller than when the erase threshold E is thereference.

In the example illustrated in FIG. 13C, the threshold of the memorycells at the end of the cells to be programmed is C; and the thresholdof the cell before being programmed adjacent thereto is E′. Thereby, thedifference of the thresholds between the memory cell at the end of thecells to be programmed and the adjacent cell is C-E′.

Here, because the distribution of the threshold E′ is shifted moretoward the thresholds A to C side (the positive polarity side) than isthe distribution of the erase threshold E, this is a state in which thedifference of the thresholds is smaller than the case where the adjacentcell is as-is at the erase threshold E; and the electrons are not easilyremoved from the storage regions of the programmed cells (referring toFIG. 7).

Then, the additional programming is performed. As illustrated in FIG.13D, the additional programming is performed on the cells to beprogrammed including the adjacent cell of the programmed cells. Here,the cells to be programmed including the adjacent cell have thethreshold E′ due to the previous E′-programming. Because “11” from thefour values is represented by the threshold E′, the programming issubstantially not performed for the cells to be programmed whenprogramming “11.”

Because the threshold E′ which has the lowest voltage of the four-valuedinformation is utilized in the E′-programming, it is sufficient for theprocessing to increase the charge injected into the storage region whenreprogramming to the other information (A to C). That is, the additionalprogramming of the desired information can be performed as-is withoutperforming the erasing operation.

Third Embodiment

An example of a nonvolatile semiconductor memory device according to athird embodiment will now be described. The nonvolatile semiconductormemory device according to the third embodiment is a nonvolatilesemiconductor memory device including the connection portion CP and theback gate BG described using FIG. 2 to FIG. 5. The descriptionshereinbelow focus on points different from the operations of thenonvolatile semiconductor memory device according to the firstembodiment.

FIGS. 14A to 14D are schematic views illustrating an example of thetransition of the information of the memory cells (part 1); and FIGS.15A to 15D are schematic views illustrating the example of thetransition of the information of the memory cells (part 2). FIGS. 14A to14D and FIGS. 15A to 15D illustrate equivalent circuits to describe thisembodiment; and the strings are simplified for description by beingillustrated as circuits extending in one direction. This does notrepresent the actual configuration.

The description herein is an operational example of a configuration inwhich the back gate BG is provided in the center of the string and themultiple memory cell transistors Tr are connected in series on one sideand on one other side centered on the back gate BG. Such a string is,for example, a NAND string having a U-shaped configuration formed of thefirst and second semiconductor pillars SP1 and SP2 and the connectionportion CP as illustrated in FIG. 3. The back gate BG is providedopposing the connection portion CP. Because the back gate BG has thesame configuration as the memory cell transistor Tr, some kind ofinformation can be programmed in the storage layer corresponding to theback gate BG as necessary. This region where the information can beprogrammed is taken to be the storage region corresponding to the backgate BG for convenience of description.

Any of the threshold distributions of the first embodiment illustratedin FIG. 9A to FIG. 11C also may be used as the erase threshold E and thethresholds A to D corresponding to the four-valued information in thenonvolatile semiconductor memory device according to the thirdembodiment. As an example, the thresholds illustrated in FIGS. 9A and 9Bare used in the description of this embodiment. The programming of thefour-valued information (the programming) is performed in order from thememory cell on the source side or the memory cell on the drain side. Theprogramming also may be performed to randomly-accessed memory cells. Thecase of the programming being performed in order from the memory cell onthe source side is described as an example in this embodiment.

First, the operations in the case where the programming of thefour-valued information stops at the end portion of the back gate BGwill be described based on FIGS. 14A to 14D.

As illustrated in FIG. 14A, the control unit CTU (referring to FIG. 2)performs the collective erasing of the multiple memory cells of thestring. By the erasing operation, the thresholds of the multiple memorycell transistors Tr are set to the threshold E. Here, for the storageregion corresponding to the back gate BG, there are cases where theerasing operation is performed and cases where the erasing operation isnot performed.

Then, the control unit CTU performs the programming of the informationto the cells to be programmed of the string. In the example illustratedin FIG. 14B, the state is illustrated in which the programming isperformed for the memory cells from the source side up to the memorycell before the back gate BG.

Continuing, the control unit CTU performs the A-programming as necessary(referring to FIG. 14C). Here, the adjacent cell on which theA-programming is to be performed corresponds to the storage regioncorresponding to the back gate BG. In the case where the erasingoperation is not performed to the storage region corresponding to theback gate BG in the previous collective erasing, the A-programming mayor may not be performed to the storage region corresponding to the backgate BG corresponding to the adjacent cell.

On the other hand, in the case where the erasing operation is performedto the storage region corresponding to the back gate BG in the previouscollective erasing, the A-programming is performed to the storage regioncorresponding to the back gate BG corresponding to the adjacent cell.Thereby, the difference of the set thresholds, i.e., the difference ofthe charge, between the memory cell at the end of the programmed cellsand the storage region corresponding to the back gate BG adjacent to thememory cell at the end is smaller than that of the state directly afterthe programming.

Then, the control unit CTU performs the additional programming(referring to FIG. 14D). The additional programming starts from thefirst memory cell after the back gate BG. The additional programming canbe executed by a method similar to the programming. Thereby, the cellsto be programmed in the additional programming have one selected fromthe thresholds A to D of the four values.

Operations of the case where the programming of the four-valuedinformation straddles the back gate BG will now be described based onFIGS. 15A to 15D.

As illustrated in FIG. 15A, the control unit CTU performs the collectiveerasing of the multiple memory cells of the string. By the erasingoperation, the thresholds of the multiple memory cell transistors Tr areset to the threshold E. Here, for the storage region corresponding tothe back gate BG, there are cases where the erasing operation isperformed and cases where the erasing operation is not performed.

Then, the control unit CTU performs the programming of the informationto the cells to be programmed of the string. In the example illustratedin FIG. 15B, the state is illustrated in which the programming isperformed from the source side up to memory cells straddling the backgate BG. Here, the programming of the information providing thethreshold A may be performed to the storage region corresponding to theback gate BG.

Continuing, the control unit CTU performs the A-programming (referringto FIG. 15C). Here, the information for which the thresholds of thememory cell transistors become the threshold A is programmed to theadjacent cell of the programmed cells on which the programming isperformed.

In this A-programming, the A-programming is performed also to thestorage region corresponding to the back gate BG as necessary. In otherwords, in the case where the erasing operation was not performed to thestorage region corresponding to the back gate BG in the previouscollective erasing, the A-programming may or may not be performed to thestorage region corresponding to the back gate BG. Further, in the casewhere the programming of the information providing the threshold A wasperformed to the storage region corresponding to the back gate BG duringthe previous programming, it is unnecessary to perform the A-programmingto the storage region corresponding to the back gate BG.

On the other hand, in the case where the erasing operation is performedin the previous collective erasing and the information providing thethreshold A is not programmed to the storage region corresponding to theback gate BG, the A-programming is performed to the storage regioncorresponding to the back gate BG corresponding to the adjacent cell.Thereby, the difference of the set thresholds, i.e., the difference ofthe charge, between the storage region corresponding to the back gate BGand the memory cell adjacent thereto, which are included in theprogrammed cells, is smaller than that of the state directly after theprogramming.

Because the additional programming described below is not performed tothe storage region corresponding to the back gate BG which is includedin the programmed cells, the width of the distribution of the thresholdA may be wider than those of the distributions of the thresholds B to Dof the other information for the programming and the A-programmingperformed to the storage region corresponding to the back gate BG.Thereby, the number of verifies of the programming and the A-programmingcan be less than the number of verifies of the thresholds B to D and theprogram time can be shortened.

Then, the control unit CTU performs the additional programming(referring to FIG. 15D). The additional programming starts from the cellof the latter stage adjacent to the series of data on which theprogramming straddling the back gate BG was performed previously and forwhich the A-programming was performed. The information having thedesired 4 values is programmed to the cells to be programmed in thelatter stage including the adjacent cell. Thereby, the cells to beprogrammed in the additional programming have one selected from thethresholds A to D of the four values.

Thus, in the nonvolatile semiconductor memory device in which the backgate BG is provided in the center of the string, this is a state inwhich the electrons are not easily removed from the storage regions ofthe programmed cells (referring to FIG. 7) because the difference of thestored charge between the cells to which the information is programmedand the storage region corresponding to the back gate BG is smaller thanthose of the states directly after the collective erasing and directlyafter the programming; and the data retention characteristics improve.

Although all of the embodiments described above are examples in whichall of the threshold distributions representing the multi-bitinformation or all but a portion are on the positive polarity side, theinvention is not limited thereto. This is similar even in the case whereall of the threshold distributions representing the multi-bitinformation or all but a portion are set to be on the negative polarityside.

Drive Circuit Configuration

FIG. 16 is a circuit diagram illustrating a drive circuit configurationof the nonvolatile semiconductor memory device according to thisembodiment. In other words, the nonvolatile semiconductor memory deviceincludes a cell array and decoders. The cell array includes n (n beingan integer not less than 1) blocks including m (m being an integer notless than 1) strings. In one string, multiple memory cells are provided;and the memory cell transistors of the memory cells are connected inseries. The thresholds of the memory cell transistors fluctuate due tothe information set in the memory cells.

The decoders are the row decoders; a decoder is provided every block ofthe cell array; and n decoders are provided. That is, a row decoder 0 isprovided corresponding to a block 0, a row decoder 1 is providedcorresponding to a block 1, . . . , a row decoder i is providedcorresponding to a block . . . , and a row decoder n is providedcorresponding to a block n.

The row decoder i connected to the block i provides signals SGD1<i> toSGDm<i> to the drain-side selection gate electrodes SGD of the m stringsof the block i and provides the signals SGS1<i> to SGSm<i> to thesource-side selection gate electrodes SGS. The row decoder i providessignals to the electrode films WL of the block i in the unit of layers.Because there are four layers of electrode films WL in the exampleillustrated in FIG. 16, signals WL1<i> to WL4<i> are provided. The rowdecoders other than the row decoder i have similar configurations; andsignals similar to those recited above are applied to the correspondingblocks.

Bit lines BL0 to BLm have common connections to the m strings of each ofthe blocks for the blocks 0 to n of the cell array; and the source lineSL has a common connection to the blocks.

Driver circuits DV1 to DV4 control the signals sent to the bit lines BL0to BLm and the source line SL and control the row decoders. The drivercircuits DV1 to DV4 are circuits configured to control the signalsWL1<i> to WL4<i> of each of the blocks 0 to n. The driver circuit DV1controls the signal WL1<i> of each of the blocks 0 to n; the drivercircuit DV2 controls the signal WL2<i> of each of the blocks 0 to n; thedriver circuit DV3 controls the signal WL3<i> of each of the blocks 0 ton; and the driver circuit DV4 controls the signal WL4<i> of each of theblocks 0 to n. The signals output from the driver circuits DV1 to DV4are sent to the signals WL1<i> to WL4<i> of each of the blocks 0 to nvia the row decoders 0 to n, respectively.

The driver circuit may be provided in the same chip as the nonvolatilesemiconductor memory device or may be provided outside the chip.

In the nonvolatile semiconductor memory device according to thisembodiment, the memory cell transistors are set to the thresholdscorresponding to the information having n values (e.g., 4 values); andin such a state, the information of the at least one cell before beingprogrammed adjacent to the memory cells in which the information isprogrammed is controlled to have a value providing a threshold of thememory cell transistor nearer than the erase threshold to the thresholdscorresponding to the information having the n values.

In the example illustrated in FIGS. 13A to 13D, the signals WL1<i> toWL4<i> to the electrode films WL sent from each of the row decoders 0 ton to each of the blocks 0 to n are controlled to be voltagescorresponding to the set thresholds by instructions from the drivercircuits DV1 to DV4.

Although the embodiments described above are examples in which thenonvolatile semiconductor memory device mainly includes a NAND stringhaving a U-shaped configuration in which two semiconductor pillars areconnected by a connection portion, this may be applied also to anonvolatile semiconductor memory device which has no connection portionand includes NAND strings having I-shaped configurations in which eachof the semiconductor pillars is independent.

This is applicable not only to the configuration in which a stackedstructural body including the electrode film WL alternately stacked withthe inter-electrode insulating film 14 is pierced by semiconductorpillars but also to, for example, a planar nonvolatile semiconductormemory device having a MONOS structure in which multiple storage regionsare provided in a continuous storage layer having a planar configurationand electrode units are formed in the storage regions with an insulatingfilm interposed therebetween.

In the nonvolatile semiconductor memory device according to thisembodiment, the inter-electrode insulating film 14, the inner insulatingfilm 42, and the outer insulating film 43 may include a single-layerfilm of one selected from the group consisting of silicon oxide, siliconnitride, silicon oxynitride, aluminum oxide, aluminum oxynitride,hafnia, hafnium aluminate, hafnia nitride, hafnium nitride aluminate,hafnium silicate, hafnium nitride silicate, lanthanum oxide, andlanthanum aluminate or a stacked film made of a plurality selected fromthe group.

The charge storage film 48 may include a single-layer film of oneselected from the group consisting of silicon nitride, siliconoxynitride, aluminum oxide, aluminum oxynitride, hafnia, hafniumaluminate, hafnia nitride, hafnium nitride aluminate, hafnium silicate,hafnium nitride silicate, lanthanum oxide, and lanthanum aluminate or astacked film made of a plurality selected from the group.

In the specification of the application, “perpendicular” and “parallel”refer to not only strictly perpendicular and strictly parallel but alsoinclude, for example, the fluctuation due to manufacturing processes,etc. It is sufficient to be substantially perpendicular andsubstantially parallel.

Hereinabove, exemplary embodiments of the invention are described withreference to specific examples. However, the invention is not limited tothese specific examples. For example, one skilled in the art maysimilarly practice the invention by appropriately selecting specificconfigurations of components included in nonvolatile semiconductormemory devices such as semiconductor substrates, electrode films,insulating films, insulating layers, stacked structural bodies, storagelayers, charge storage layers, semiconductor pillars, word lines, bitlines, source lines, interconnects, memory cell transistors, selectiongate transistors, and the like from known art. Such practice is includedin the scope of the invention to the extent that similar effects theretoare obtained.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility and are included inthe scope of the invention to the extent that the purport of theinvention is included.

Moreover, all nonvolatile semiconductor memory devices practicable by anappropriate design modification by one skilled in the art based on thenonvolatile semiconductor memory devices described above as embodimentsof the invention also are within the scope of the invention to theextent that the purport of the invention is included.

Furthermore, various modifications and alterations within the spirit ofthe invention will be readily apparent to those skilled in the art. Allsuch modifications and alterations should therefore be seen as withinthe scope of the invention. For example, additions, deletions, or designmodifications of components or additions, omissions, or conditionmodifications of processes appropriately made by one skilled in the artin regard to the embodiments described above are within the scope of theinvention to the extent that the purport of the invention is included.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modification as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A nonvolatile semiconductor memory device,comprising a memory unit and a control unit, the memory unit including aplurality of strings, the strings having: a charge storage film; and amemory cell transistor provided for each of a plurality of storageregions configured to store charge in the charge storage film, athreshold of the memory cell transistor fluctuating due to data set inthe storage region, the control unit being configured to: set the memorycell transistors to an erase threshold by setting erase data in theplurality of storage regions of at least one of the strings;subsequently set the memory cell transistors provided in the storageregions to thresholds corresponding to data having n (n being an integernot less than 2) values by programming the data having the n values toat least one of the storage regions of the strings in which the erasedata is set; and control data of at least one storage region beforebeing programmed adjacent to the storage regions programmed with thedata to have a value providing a threshold of the memory cell transistornearer than the erase threshold to the thresholds corresponding to thedata having the n values in the state of the memory cell transistorsprovided in the storage regions being set to the thresholdscorresponding to the data having the n values.
 2. The device accordingto claim 1, wherein the control unit is configured to control all of thethresholds of the transistors due to the data of the programmed storageregions and the threshold of the transistor due to the data of the atleast one storage region before being programmed adjacent to theprogrammed storage regions to have one selected from a positive polarityand a negative polarity.
 3. The device according to claim 1, wherein thecontrol unit is configured to control the data of the at least onestorage region before being programmed adjacent to the programmedstorage regions to have a value providing a threshold of the memory celltransistor contained in a distribution straddling 0 V.
 4. The deviceaccording to claim 1, wherein the control unit is configured to controlthe data of the at least one storage region before being programmedadjacent to the programmed storage regions to have a value providing thethreshold nearest to the erase threshold of the thresholds of the memorycell transistors corresponding to the data having the n values.
 5. Thedevice according to claim 1, wherein the control unit is configured tocontrol the data of the at least one storage region before beingprogrammed adjacent to the programmed storage regions to have a valueproviding a threshold of the memory cell transistor contained in a rangenarrower than a distribution of the erase threshold and wider thandistributions of the thresholds corresponding to the data having the nvalues.
 6. The device according to claim 1, wherein the control unit isconfigured to set data in the plurality of storage regions to provide athreshold on a positive polarity side of the erase threshold aftersetting the memory cell transistors to the erase threshold by settingthe erase data in the plurality of storage regions and prior toprogramming the data having the n values to the storage regions.
 7. Thedevice according to claim 6, wherein the data providing the threshold onthe positive polarity side of the erase threshold set by the controlunit is one selected from the data having the n values.
 8. The deviceaccording to claim 6, wherein the control unit is configured to set adistribution of the threshold on the positive polarity side of the erasethreshold to be narrower than a distribution of the erase threshold. 9.The device according to claim 1, wherein the control unit is configuredto set the thresholds of the memory cell transistors to the erasethreshold collectively for each of the strings.
 10. A method for drivinga nonvolatile semiconductor memory device, the device including aplurality of string, the strings having: a charge storage film; and amemory cell transistor provided for each of a plurality of storageregions configured to store charge in the charge storage film, athreshold of the memory cell transistor fluctuating due to data set inthe storage regions, the method comprising: setting the memory celltransistors to an erase threshold by setting erase data in the pluralityof storage regions of at least one of the strings; subsequently settingthe memory cell transistors provided in the storage regions tothresholds corresponding to data having n (n being an integer not lessthan 2) values by programming the data having the n values to at leastone of the storage regions of the strings; and controlling data of atleast one storage region before being programmed adjacent to the storageregions programmed with the data to have a value providing a thresholdof the memory cell transistor nearer than the erase threshold to thethresholds corresponding to the data having the n values in the state ofthe memory cell transistors provided in the storage regions being set tothe thresholds corresponding to the data having the n values.
 11. Themethod according to claim 10, wherein data having a value providing athreshold nearer than the erase threshold to the thresholdscorresponding to the data having the n values is set to the storageregion before being programmed after the data having the n values isprogrammed to the storage regions.
 12. The method according to claim 10,wherein the data of the plurality of storage regions is controlled to bea value providing a threshold of the memory cell transistor nearer thanthe erase threshold to the thresholds corresponding to the data havingthe n values after the setting of the erase data in the plurality ofstorage regions and prior to programming the data having the n values tothe storage regions.
 13. The method according to claim 10, wherein allof the thresholds of the transistors due to the data of the programmedstorage regions and the threshold of the transistor due to the data ofthe at least one storage region before being programmed adjacent to theprogrammed storage regions are controlled to have one selected from apositive polarity and a negative polarity.
 14. The method according toclaim 10, wherein the data of the at least one storage region beforebeing programmed adjacent to the programmed storage regions iscontrolled to have a value providing a threshold of the memory celltransistor contained in a distribution straddling 0 V.
 15. The methodaccording to claim 10, wherein the data of the at least one storageregion before being programmed adjacent to the programmed storageregions is controlled to have a value providing the threshold nearest tothe erase threshold of the thresholds of the memory cell transistorscorresponding to the data having the n values.
 16. The method accordingto claim 10, wherein the data of the at least one storage region beforebeing programmed adjacent to the programmed storage regions iscontrolled to have a value providing a threshold of the memory celltransistor contained in a range narrower than a distribution of theerase threshold and wider than distributions of the thresholdscorresponding to the data having the n values.
 17. The method accordingto claim 10, wherein data is set in the plurality of storage regions toprovide a threshold on a positive polarity side of the erase thresholdafter setting the memory cell transistors to the erase threshold bysetting the erase data in the plurality of storage regions and prior toprogramming the data having the n values to the storage regions.
 18. Themethod according to claim 17, wherein the data providing the thresholdon the positive polarity side of the erase threshold is controlled to beone selected from the data having the n values.
 19. The method accordingto claim 17, wherein a distribution of the threshold on the positivepolarity side of the erase threshold is controlled to be narrower than adistribution of the erase threshold.